In general, transistor linearity is defined in terms of a linearity figure-of-merit (LFOM) of the output third-order intercept point OIP3/PDC. Most existing semiconductor technologies are limited to a LFOM of 10 dB. Exceptions include doped-channel GaAs MESFETs with a LFOM of ˜50 dB, as described in Reference 1 below, which is incorporated herein by reference, graded-channel GaAs MESFETs with a LFOM of 128 dB after load match, as described as described in Reference 2 below, which is incorporated herein by reference, and double (i.e., front-side and back-side) pulse-doped InP HEMTs with a LFOM of 41 dB, as described in Reference 3 below, which is incorporated herein by reference. These LFOM improvements are due to transistor structures with a distributed doping profile, resulting in a reduction in nonlinearities of gm and Cgs. Unfortunately, these GaAs MESFETs have degraded channel mobility, transconductance, and noise figures, all of which limit their practical use in low-noise and high-linearity receiver applications. Double-pulse-doped InP HEMTs can offer high fT/fmax and low noise at millimeter-wave (mmW) frequencies, but their LFOM degrades quickly above a −20 dBm power level due to their low breakdown voltage. This degradation limits their use in receiver applications with a high peak-to-average power ratio (e.g., 10.6 for Wideband Code Division Multiple Access (WCDMA) and ˜12 for Orthogonal Frequency Division Multiplexing (OFDM) and for in-band interferers). The highest reported LFOM in HBT devices is ˜11 dB for a GaAs HBT due to the nonlinearities from transconductance and base-collector capacitance, as described in Reference 4 below, which is incorporated herein by reference. These HBT devices also typically have a higher noise figure than HEMT devices. The reported LFOM of conventional GaN HEMTs, as described in Reference 5 below, which is incorporated herein by reference, GaN FINFETs, as described in Reference 6 below, which is incorporated herein by reference, and carbon nanotube FETs, as described in Reference 7 below, which is incorporated herein by reference, are less than 10 dB.
Recently, GaN FETs with graded AlGaN channels have been reported to show a promising linearized gm over gate voltage, though no measured linearity data was reported, and the reported device transconductance was low, 93 mS/mm or ˜159 mS/mm, as described in References 8 and 9 below, which are incorporated herein by reference. These devices also have a low mobility (524 cm2/Vs).
Most importantly, the best LFOMs of prior art semiconductor technologies are obtained around ˜0.5 Ids, while the best noise figures are obtained near ˜0.2 Ids bias point due to the channel temperature rise. Hence, the device linearity versus noise figure of the prior art transistors has been inevitably compromised.
A composite-channel GaN HEMT has been described in Reference 10 below, which is incorporated herein by reference, and has shown that double-channel GaN heterostructures improve the access resistance. However, the authors describe removing the top channel within the channel area, while keeping the double-channel in the source and drain ohmic areas, making the active channel area of this device effectively that of a signal-channel GaN HEMT.